The present invention concerns a bituminous soundproofing sheet provided on its underside with a bitumen-compatible hot-melt adhesive and containing magnetic or magnetizing particles.
Illustratively, soundproofing sheets are known in the art and are used in automobiles to soundproof body parts. Known products are characterized in that either their construction is homogeneous with uniform distribution of the magnetic particles, or they are built up in a sandwich construction in the manner of a magnetic and a soundproofing layer. On the top side, such sheets may be laminated or coated with antiblocking means. To improve soundproofing, the bitumen used as a rule is modified with plastics or synthetic resins and organic filters. The magnetic particles, which can be for instance ferrite powder, allow preliminary positioning of the soundproofing sheet during assembly and fixing it in place until there is fusion with the support and renewed bonding of the hot-melt adhesive while passing through the oven wherein the temperatures may rise up to 190.degree. C.
An example of such prior known construction is in U.S. Pat. No. 3,243,374.
This known soundproofing sheet suffers from the drawback that air bubbles trapped between the hot-melt adhesive layer and the lower support layer can expand unhampered on account of the high temperatures and lowered viscosities of the bituminous substances, whereby good bonding between the sheet and the supporting layer is hampered in part. This affects the soundproofing capability of the sheet adversely.
Accordingly, a problem had to be overcome in developing a magnetically adhering bituminous soundproofing sheet that would be free from bubble formation even after passing through the oven.